This invention relates to a liquid crystal display device and, more particularly, to a liquid crystal device of a drive circuit built-in type in which a display pixel portion and a drive circuit portion are incorporated integrally on a common substrate.
A drive circuit built-in liquid crystal display device integrally incorporating its drive circuit on a glass substrate is under progressive researches and developments toward its practical use because it leads to a reduction of components, simplifies the process for packaging the drive circuit onto the liquid crystal display panel, and hence contributes to a reduction of the cost.
A drive circuit built-in liquid crystal device is typically made, in case of TFT-LCD, by enclosing a liquid crystal between an array substrate having thin-film transistors as switching elements arranged in a matrix in accordance with pixels and an opposite substrate having formed color filters, providing polarization plates to these substrates, respectively, and mounting an illumination back light behind them. The matrix-arrayed substrate includes a display pixel portion made up of scanning lines and signal lines which are aligned in form of a matrix on the glass substrate and liquid crystal pixels formed on their crossing points via thin-film transistors as switching elements, and a peripheral drive circuit which is made simultaneously with the thin-film transistors in a common manufacturing process to surround the display pixel portion. The peripheral drive circuit includes a scanning line drive circuit for controlling switching actions of the thin-film transistors connected to pixels and a signal line drive circuit for supplying video signals to the thin-film transistors via the signal lines.
The signal line drive circuit includes a group of analog switches responsive to a timing signal for selectively connecting video signal lines to signal electrodes to supply video signals, and must operate in a higher frequency than the scanning line drive circuit. Moreover, along with progressively increasing demands for high fidelity, large capacity display, and so on, in high vision using an increased number of pixels, there have been remarked problems such as insufficient transmission bands of the video bus lines for transmitting video signals within the signal line drive circuit, and insufficient writing capacity of the analog switches for sampling the video signals on the video bus lines and supplying them to pixel switching elements.
To cope with the problems, a conventional technique divides the signal line drive circuit into a plurality of blocks and effects sampling of analog switches simultaneously within the blocks to lower the operation frequency. That is, by dividing the video bus lines into some blocks to introduce video signals in parallel and by having analog switches connected to each block of the video bus lines via a connection wiring to operate collectively for sampling, the operation frequency can be lowered by the number of blocks of the video bus so as to compensate the insufficient writing capacity of the analog switches.
However, when the conventional drive circuit built-in liquid is crystal display device is configured to supply video signals to a plurality of divisional blocks of the video signal lines as explained above, there arises the problem that stripe-shaped imaging defects 2 (stripe-shaped defects) extending longitudinally (in the column direction) appear on the display screen 1 as shown in FIG. 7, and degrade the imaging quality.
The Inventors made researches to locate its reason, and found a strong relation between the positions of the imaging defects on the screen and the positions of connection between analog switches and the video buses.
More specifically, immediately after sapling by an analog switch, electric charges stored in the analog switch flow in toward a video bus and the signal line connected to the analog switch. The flow of the electric charge causes the potential on the signal line to shift, and hence causes the signal written in a liquid crystal pixel to slightly shift from the video signal on the video bus.
In an analog switch connected to a video bus located far from the display pixel portion, the connection wiring between the analog switch and the video bus is longer and results in increasing the resistance of the connection wiring. As a result, electric charges accumulated in the analog switch during sampling is difficult to flow toward the video bus, and the ratio of the charges flowing toward the signal line increases.
In contrast, in an analog switch connected to a video bus near the display pixel portion, the connection wiring is shorter, the wiring resistance is lower, and the ratio of electric charge accumulated in the analog switch and flowing toward the signal line decreases.
This results in the phenomenon that shift amounts of video signals are small in signal lines connected to analog switches with shorter connection wirings to video buses whereas shift amounts of video signals are large in signal lines connected to analog switches with longer connection wirings. Therefore, effective voltage values applied to liquid crystal pixels vary with positions of signal lines, and cause them to vary in transmittance.
The Inventors found that, since the arrangement of connection points of analog switches and video buses was repeated for every block of sampling circuits, the difference in transmittance of the liquid crystal pixels was produced on the screen periodically along the row direction, and was noticeable as imaging defects appearing in the column direction.
FIG. 6 shows a wiring pattern in a signal line drive circuit by a conventional approach.
In FIG. 6, video signals SV1 to SV6 are applied to video buses 101 to 106 in this order. These video buses 101 through 106 and analog switches SW are connected in this order by connection wirings 211 to 216 via contact holes. As a result, adjacent signal electrodes are supplied with signals from adjacent video buses. Since lengths in of connection wirings are different only by the distance S between their video buses, difference in capacities caused by the wiring resistance and crossing of wirings is small, and no image noise occurs there.
However, in case of this example for comparison, there is a large difference in length of the connection wiring at the position where the shift register is switched from a certain stage to another. That is, the last wiring to the first stage (SR11) of the shift register and the first wring to the second stage (SR21) are different in length as large as 5 pitches, and the difference is as large as five times the difference in length between other adjacent wirings. Thus, the difference in wiring resistance is large and causes the difference in shift amount of video signal mentioned above.
Therefore, in the conventional device, the load to the wiring changes largely at the position where the shift register is switched from a stage to another, and image noise such as imaging defects cannot be prevented.
It is therefore an object of the invention to provide a liquid crystal display device of having a built-in drive circuit, which alleviates imaging defects caused by changes in length of wirings and improves the imaging quality.
According to the first aspect of the present invention, there is provided a liquid crystal display device having:
a display pixel portion including a plurality of liquid crystal pixel cells arranged in a matrix on an insulating substrate and a plurality of signal lines each connected commonly to said liquid crystal pixels in a column; and
a signal line drive circuit including groups of positive-polarity video buses for transmitting positive-polarity video signals, groups of negative-polarity video buses disposed in parallel with said groups of the positive-polarity video buses to transmit negative-polarity video signals, and sampling circuit blocks made up of a plurality of positive-polarity switches connected individually to one of said positive-polarity video buses via connection wirings and a plurality of negative switches connected individually to one of said negative-polarity video buses via connection wirings so that both said switches align between said groups of the video buses and of the display pixel portion to make switch pairs each including one of said positive-polarity switches and one of said negative-polarity switches which are connected to a common said signal line,
arrangement of connection points of said connection wirings of said positive-polarity switches to said positive-polarity video buses in a said sampling circuit block being substantially symmetric with arrangement of connection points of said connection wirings of said negative-polarity switches to said negative-polarity video buses in the same sampling circuit block with respect to a border line between said positive-polarity video buses and said negative-polarity video buses.
According to the invention, imaging defects can be reduced by an improved arrangement of connection points of sampling switches and video bus lines in the signal line drive circuit, in which connection points of a block of video buses supplied with video signals of the positive polarity relative to a predetermined reference potential and a block of video buses supplied with video signals of the negative polarity to their associated analog switches are arranged substantially symmetrically in the extending directions of the video.
More specifically, in the liquid crystal display device according to the invention, since connection points are disposed so that their arrangement of the positive polarity switches and that of the negative polarity switches be symmetric, if a switch of one polarity in a particular pair of switches has a long connection wiring, then the other switch of the other polarity has a short connection wiring. In other words, the sum of connection wirings of a pair of switches and their resistance value are substantially equal to the sum of connection wirings of another pair of switches and their resistance value. As a result, the effective value of the shift amount of the signal line potential is substantially equal in all signal lines, and imaging are alleviated.
According to the second aspect of the present invention, there is provided a liquid crystal display device having:
a display pixel portion including a plurality of pixel capacitors arranged in a matrix on an insulating substrate and a plurality of signal lines each connected commonly to said pixel capacitors in a column; and
a signal line drive circuit including positive-polarity video buses for transmitting positive-polarity video signals and negative-polarity video buses for transmitting negative-polarity video signals which are aligned alternately, and sampling circuit blocks made up of a plurality of positive-polarity switches connected individually to different said positive-polarity video buses via connection wirings and a plurality of negative switches connected individually to different said negative-polarity video buses via connection wirings so that both said switches align between said video buses and said display pixel portion so as to make switch pairs each including one of said positive-polarity switches and one of said negative-polarity switches which are connected to a common said signal line,
the sum of resistances of connection wirings of said positive-polarity switch and said negative-polarity switch which make a switch pair being substantially constant for all said switch pairs within said sampling circuit block.
According to the third aspect of the present invention, there is provided a liquid crystal display device having same elements as the second aspect except for a relation where the sum of lengths of connection wirings of said positive-polarity switch and said negative-polarity switch which make a switch pair being substantially constant for all said switch pairs within said sampling circuit block.